The present invention relates to an image display device using transistors each having a polycrystalline semiconductor layer, as well as to a manufacturing method of such image display device.
For example, a liquid crystal display device includes pixel areas each made of an area surrounded by a pair of gate signal lines and a pair of drain signal lines on a liquid-crystal-side surface of one of substrates disposed in opposition to each other with a liquid crystal interposed therebetween, and each of the pixel areas is provided with a thin film transistor and a pixel electrode. The thin film transistor is operated by a scanning signal from one of the pair of gate signal lines, and the pixel electrode is supplied with a video signal from one of the pair of drain signal lines via this thin film transistor.
As such thin film transistor, a thin film transistor having a semiconductor layer using, for example, a polycrystalline Si (poly-Si) is known.
The semiconductor layer is formed by forming a layer made of amorphous Si (a-Si) on a substrate surface and polycrystallizing the layer by, for example, irradiation with laser light.
The thin film transistor is also formed by adding, for example, boron (B) to the polycrystallized semiconductor layer to make it P-type, forming an oxide film on a surface of the P-type semiconductor layer, forming a gate electrode on the upper surface of this oxide film in such a manner that the gate electrode intersects the semiconductor layer, and adding, for example, phosphorus (P) by using the gate electrode as a mask, to make the semiconductor layer N-type on the opposite sides of an area immediately below the gate electrode, i.e., in a drain region and a source region.
Incidentally, during the formation of the drain region and the source region, it is general practice to add phosphorus (P) with a mask (photoresist) withdrawn and form a so-called LDD region (Lightly Doped Drain) as an N-type region at the boundary between each of the drain region and the source region and the area immediately below the gate electrode.